1. Field of the Invention
This invention relates generally to apparatus useful for measuring fluid flow. In particular, it relates to improvements in apparatus for monitoring well down-hole conditions, particularly to a monitoring device for measuring the flow of mud, and other drilling-fluid parameters relating to flow in real time in the drilling of oil and gas wells.
2. Problems and Description of the Prior Art
Rotary drilling, as practiced in oil and gas production, requires the formation of a hole, or well bore extending downwardly from the earth's surface to an oil producing stratum. Formation of the well bore requires generally a casing extending from the earth's surface downwardly with continued cutting into the earth's surface with a rotating bit attached to the end of a drill pipe string to which joints of pipe are sequentially attached as the well bore is extended from the surface downwardly. In the drilling operation, a drilling fluid, or mud, constituted generally of a mixture of weighting materials, clays, chemicals, and water or oil, is removed from a mud pit and pumped down the drill string into the sealed well bore to exit through jets in the drill bit at the bottom of the hole, the fluid or mud being recycled, ascending to the surface via the annular space between the exterior wall of the drill string and the wall of the hole, or well bore. At the surface, the mud received from the casing flows to a shale shaker for cuttings removal, and it is then returned to the mud pit for treatment and storage for further use.
The drilling mud serves several essential functions, the most important of which are to seal off permeable formations to prevent loss of drilling fluid as the well is drilled through different subterranean formations, lubricate the drill bit and drill string, provide a hydrostatic head to restrain the flow of high pressure oil, gas or water from subterranean formations into the well bore, remove cuttings from the well and, in the event of a shutdown in the drilling operation, to hold the cuttings, sand and other residual materials in suspension within the static column of drilling mud. Substantial quantities of clays and other colloidal materials are added by the mud engineer to assist in imparting the required density, viscosity and gel strength to the mud as required for the entrainment and suspension of the cuttings. This is necessary because down-hole conditions creates these, and other physical and chemical changes in the mud. Gas, water and oil also become components of the mud, which is or becomes a multiple phase slurry constituted of liquids, solids and gases. The rheological, or flow properties of the mud during use thus invariably change due to additions made to the mud by the mud engineer, and because of various conditions which produce undesired changes in the thixotropic properties of a mud, this making it mandatory to constantly treat the mud to maintain the desired density and thixotropy. It is imperative that the mud be sufficiently fluid that it can be pumped, and sufficient hydrostatic pressure must be maintained by the column of mud to prevent escape of gas or oil from the surrounding strata as the depth of the well bore is extended into the earth.
It is further and particularly imperative during drilling operations that the operator monitor down-hole conditions. The operator, in particular, must have information relating to changes in the circulation, or flow of the drilling mud, and he must have it as soon as possible, to make intelligent operational and procedural decisions relating to the drilling operation. Of vital importance, the operator must know the amount of mud that is introduced into the well bore, and the amount of mud that is returned to the surface from the well bore. Circulation changes do occur due, e.g. to gas pockets, structure crumbling, and the like; and these are often unsafe conditions. A lesser flow of mud from the well bore than the input of mud into the well bore may thus indicate "loss-of-circulation", a phenomenon wherein drilling fluid is lost into a formation cavity. Under such circumstances the well can be lost due to insurmountable economic costs. Conversely, a higher flow of mud from the well bore than the flow of mud into the well bore may indicate that fluids are being transported from the surrounding strata into the well bore. This effect could be caused by anything from salt water to high pressure gas. A high pressure gas pocket can cause the loss of several thousand feet of drill pipe, and it could indicate an impending absolutely disasterous occurrence. It may thus indicate an impending "blow-out". This latter condition can not only produce an economic disaster through complete loss of the well, equipment and drilling rig itself, but can also result in serious injuries and the loss of human lives.
Whereas attempts have been made over many years to measure the gain or loss of mud circulation in real time, such attempts have never been very successful. Thus, flow meters have been used to measure mud input, and output, one value being subtracted from the other to determine net mud flow. A principal difficulty with flow meters has been their inability to accurately measure flow rate ranges on the order of 150 or 160 to 1, as is required. Moreover, albeit the mud input to the well may be relatively homogenous, the return flow of mud is all but homogenous. With regard to mud input, or mud output, the wide range of changes in the density of the mud, from about 0.75 to about 2.65 times the specific gravity of water, and changes in viscosity, from about 1 to about 100 centipoises, caused by the necessary addition of weighting materials, chemicals and clays to tailor the density and viscosity of the mud to changing down-hole conditions adds to the complexity of the problem. Measurement of flow, and the rate changes in flow of mud is further compounded in that there are flow fluctuations, and pulsating flow since the mud is necessarily pumped into and thus out of the well bore. In addition there are changes in fluid vapor pressure, critical pressure, temperature, and the proportion of gas, vapor and solids present in the mud. The return flow of mud is far more difficult to measure than the input flow because in addition to these problems, there is little hydrostatic pressure, head, or net energy, to perform flow measurement. Gases and solids are admixed with liquids, and the pipe returning the multiphase slurry is not necessarily filled. Further, the mud may have a water base or oil base, and this is subject to change, which in itself creates many problems which adversely affect the accuracy, or operability of flow meters. For example, the presence of even a small amount of oil in mud can relatively quickly decommission a magnetic flow meter by laying down a film on the electrodes, this resulting in a drastic loss of accuracy in measuring the rate of flow of the mud.
Another very different, and very difficult set of problems is introduced on semi-submersible or floating drilling rigs by the tide and wave motions of the sea. However, the problem that overwhelms all other motion problems in ocean drilling relates to the erratic flow and changes in the rate of flow of the return mud caused by vessel heave. This is because the mud of non-homogenous consistency within the riser slip joint located adjacent the bottom of the vessel is suddenly deaccelerated as the vessel rises, and then suddenly accelerated in various directions different from the net direction of flow as the vessel carrying the rig descends; this occurring while the vessel is rolling, pitching and yawing. The system behaves much in the manner as a large positive displacement pump. The magnitude of the problem in measuring the flow in such system of multiphase muds can be appreciated when it is realized that seven to nine foot waves are created, as in the Gulf of Mexico with very little wind, and that fifty to sixty foot waves are common in the North Sea, with even higher wave action during stormy weather. The result is that when the vessel is descending the mud is pumped very rapidly as a great slug of mud through the chamber formed by the riser slip joint. Conversely, at a moment when the vessel is rising the flow is drastically slowed, or may be completely interrupted. Thus, for several moments there is a great slug flow of mud passing through the chamber, and shortly thereafter there is no flow at all. A device or method for the adequate measurement of pulsating flows produced in this manner has been needed since the introduction of floating drill platforms, and while many have recognized the problem, satisfactory solutions have not been forthcoming.
It is nonetheless a primary objective of the present invention to obviate these and other prior art problems.
A particular objective of the present invention is to provide apparatus, apparatus combinations, and process, for measuring to a high degree of reliability, accuracy, and precision the rate of flow, differential flow rates and other parameters of a fluid, notably a drilling mud, used during such oil and gas well drilling operations.
Moreover, it is an object to provide apparatus, apparatus combination, and process for measuring or calculating the viscosity, density, temperature, and the rate of flow of a fluid, notably the drilling mud passing downwardly through the drill pipe string, the viscosity, density, temperature and rate of flow of the return drilling mud ascending to the earth's surface from within the well bore, any differences in the density and flow rate between the mud introduced into the well bore and the mud returning from the well bore, and a running total of these differences.
Another object is to provide apparatus, and an apparatus combination of the character described which is fully compensated for variations in pressures, temperature, density, viscosity, gas content and atmospheric conditions; which apparatus, or apparatus combination, is self cleaning, self calibrating and self checking for abberations or abnormalities, and which can automatically prevent flashing and cavitation conditions within the primaries which might be detrimental or harmful to measurements.
A further objective is to provide apparatus, and apparatus combinations as characterized, which measures water base and hydrocarbon base fluids with equal accuracy as well as mixtures of water, hydrocarbons, liquids, gases and solids, and includes as well added compensation for use on off-shore semi and floating vessels.
A yet further, and more specific object is to provide apparatus, and apparatus combinations as characterized, for off-shore floating or semi-floating vessels which measures vessel motion and compensates for wave and tide motions.